Sedimentation in the Upper Reaches of Monterey Canyon

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Feb 21, 2014 (3 years and 1 month ago)

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Sedimentation in the Upper Reaches of Monterey Canyon
Meagan Eagle, Stanford University
Principal Investigators: H. Gary Greene, Norman Maher and Charlie Paull
Summer 1999
Abstract
Core OO99-21 contains an as yet chronologically unbounded record of
sedimentation in the upper reaches of Monterey Canyon. Through a careful analysis of
the sedimentary structures, variations in grain size, differences in mineralogy, and
sediment carbon content of the core, it is possible to determine the nature of
sedimentation in this part of the canyon. Characterizing the sediment throughout the core
is an important step to understanding the environmental mechanisms responsible for
deposition in the near-shore Canyon head. Several techniques have been used to log the
core’s physical characteristics, including Gamma Ray (GRAPE), photographs, XRD
mineralogy, and Total Carbon content. Sedimentation of fine silts and clays appears to
dominate in the head of Monterey Canyon with occasional high energy events inputting
coarser material into the system.
Introduction
Figure 1: Monterey Canyon
Deep Sea Geology
The Monterey Canyon cuts through the continental shelf in the heart of Monterey
Bay, California (36°45’N, 121°45’W.) The California coast lies on an active tectonic
transform margin and has a narrow continental shelf. The coasts straddling Monterey
Bay are mountainous and dissected by many streams and rivers. The climate is dry in the
summer and winters are wet with frequent storms. The El Niño event has a clear signal
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in California’s climate, increasing rainfall and storms. The Monterey Canyon is an
impressive submarine feature, running many kilometers west out to sea (Fig. 1).
The Canyon’s head can be found at Moss Landing Harbor, just offshore of the
breakers at the jetty mouth. The Canyon runs from the jetty mouth to great depths,
approximately 4000 meters below the sea surface far out on the abyssal plain. Typical
relief in the upper reaches of the canyon is on the order of 100 meters or more. The
canyon width increases as it stretches into deeper waters. Many have compared the
Monterey Canyon to the Grand Canyon, which has similar relief and length (Fig. 2). But
the submarine Monterey Canyon is hidden below the depths of the cold, fertile Monterey
Bay. The study of this Canyon and the processes that have formed it and continue at
work today is much more difficult than encountered by the studies of similar terrestrial
features. The Canyon can only be studied remotely, through such classical geological
methods as coring and bottom sampling from a research vessel.
Figure 2: Relief of Grand Canyon and Monterey
Canyon. After Shepard & Dill (1966)
Recently the Monterey Bay Aquarium Research Institute (MBARI) employed
advanced technology to enable a more visual approach to submarine geology.
Employing Ventaña, a Remotely Operated Vehicle (ROV,) from the R/V Point Lobos,
geologists have been able to view the Canyon first-hand. Conditions in the deep sea still
limit the techniques and research scientists can do. Beyond 100 meters all natural light is
absorbed, and visibility using artificial light within the canyon can be anywhere from 3 to
30 meters. Ventaña is attached by a cable to the R/V Point Lobos, and can only move
slowly. The environment is harsh as well, with corrosive seawater and high pressures.
Combining geologic techniques with MBARI’s new technology, geologists have been
able to provide one of the most comprehensive pictures yet of a submarine canyon.
Monterey Canyon History
The Monterey Canyon lies within a complex geologic setting along the coast of
California (Fig. 3). The Canyon cuts through several distinct rock formations. Basement
rocks consist of Cretaceous granite from the Salinian Block and metamorphic rocks west
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of the Salinian Block (Greene et al., 1989). Above this basement are younger Tertiary
sedimentary rocks. The Monterey Bay Fault Zone runs north/south through the bay, and
influences canyon morphology (Greene et al., 1991). Clues to the formation of
Monterey Canyon may lie with these tectonically active faults. At some point in the past
100 million years, the age of the oldest basement rock the canyon cuts through, the entire
block that the canyon cuts into would have been further to the south, carried to its current
location as the Pacific Plate slid North. During the Miocene, the canyon may have been
subaerially exposed by tectonic uplift or lower sea levels (Greene et al., 1989). Faulting
within the basement rocks of the proto-canyon could have focused drainage into what is
now Monterey Bay. A canyon was created, erosion cutting into the basement rock, and
eventually the entire area was flooded with seawater. Since then the canyon has
continued to evolve, in turns eroding and depositing sediment along its length.
Figure 3: Geology of Monterey Bay (After McCulloch and Greene, 1990)
Submarine Sedimentation
For canyon topography to remain geomorphically sharp, it is necessary for
erosion to exceed sedimentation rates. The canyon cycles between stages where
sedimentation will fill in the canyon for a time and then erosional events will exhume
part of the canyon. There are several general theories on how this can occur. First, the
main causes of erosion and sedimentation are usually two phases of the same event or
phenomena. Sediment can be suspended and transported only when the system contains
adequate energy and sedimentation occurs with a loss of energy. Some of these energy
events have been recognized and include turbidity currents, density flows and currents,
debris flows and storms and mass wasting of the canyon wall. (Hampton 1972; Klein
1980; Bouma 1962; Stow 1985) Another important mechanism is pelagic accumulation,
or sedimentation of biologic debris on the sea floor. Usually the entire floor is covered
with this gray/green ooze, unless currents are so great that accumulation cannot occur.
As seen on ROV Ventaña dives, fine sediment covers much of the head of Monterey
Canyon. All sedimentation methods other than pelagic accumulation usually occur as
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distinct events and are not continuous. Each event has certain characteristics that
distinguish it from others, and as sediment accumulates, a record of these events is
deposited and buried further with each passing season. Geologists use these sediment
layers to identify modes of transportation and to ultimately piece together a complete
history of sedimentation in an area.
In a submarine canyon, accumulation of a long sediment record is difficult to
establish, especially if the canyon is actively being exhumed. In that case the entire
sediment record would be carried down the canyon to deposit on the distal reaches of the
submarine fan. For a sediment record to exist, the energy of the system must be low
enough to allow active accumulation.
Figure 4: Ventaña position on May 5, 1999 while diving on the layered sediments.
Such a sediment record exists near the head of the Monterey Canyon. Just
upstream of the bend in the Canyon known as Gooseneck meander, the lower canyon
wall is topped with a smooth ledge. Preliminary site investigation in this area was done
by H. Gary Greene with the ROV Ventaña. Dives 1600, 1601 and 1602 on May 4 and 5,
1999, were recorded on video cameras aboard the R/V Point Lobos during Ventaña’s
dives and clearly show some layering in the exposed canyon wall (Fig. 4.)
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Ventaña
collected samples from the exposed sediments. Other evidence for layered sediments can
be seen in reflective images taken of the Monterey Canyon flank with a 3.5 kHz sub-
bottom seismic reflection profiling system (Fig. 5) by H. Gary Greene from the R/V Point
Sur in March 1999. Layering within the first ten or twenty meters is visible throughout
the flat lying sediments up-canyon of Gooseneck meander in the upper reaches of
Monterey Canyon.

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MBARI address: 7700 Sandholdt Road, Moss Landing, 95039. Exact video location is: May 5, 1999
Dive 1602, PI Charlie Paull, 1.02.15 to 1.02.45 hours.
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Figure 5: 3.5 kHz image of layered sediments.
Materials and Methods
Geologists from MBARI began a project to study this sediment record in further
detail. In June 1999, the study was begun in earnest with the collection of several cores
of sediment directly from the smooth flank of Monterey Canyon. Through multiple
analyses of this core, it is anticipated that the history of sedimentation in the upper
reaches of the Monterey Canyon will be established, and the main mechanisms for
sediment transport and the implications for paleo-environment from late Tertiary to
recent time will be identified.
Core OO99-21
In June 1999, Charlie Paull and H. Gary Greene collaborated with the USGS for a
coring cruise on the M/V Ocean Olympus off the central California coast. A team of
USGS Marine Technicians and scientists ran coring operations and the geophysical core
logging (GRAPE) lab, while MBARI scientists Norman Maher, H. Gary Greene, Charlie
Paull and Bill Ussler ran the precision navigation and chemistry labs. Five cores were
successfully recovered from the layered sediments in the upper reaches of the Monterey
Canyon (Table 1, Fig. 6). These cores were collected with a piston coring method. A
core tube lined with a clear plastic core holder is lowered to the bottom. At the tip is a
metal cutter, which penetrates the sediment, with a catcher behind it that acts as a claw,
opening its tongs to let the mud through but closing together when the pressure reverses
and the sediment (mud) tries to flow out. A trigger lowers in front of the main core and
releases a 750 kilogram weight about 10 meters from the bottom. The core tube is forced
into the sediment by gravity falling and the cores falls into a position that creates a
vacuum, allowing the sediment to flow easily into the tube.
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Core Latitude Longitude Water Depth
(meters)
Core Length
(meters)
OO99-21 36 ° 47.4302 121 ° 53.2781 385 4.87
OO99-22 36 ° 47.3415 121 ° 53.8428 394 5.67
OO99-23 36 ° 47.3808 121 ° 54.3563 479 4.48
OO99-24 36 ° 47.8044 121 ° 52.9345 404 4.24
OO99-25 36 ° 47.6650 121 ° 53.3270 398 3.56
Table 1: Ocean Olympus Cruise cores collected at the head of Monterey Canyon.
Immediately upon retrieval of the core, it was cut into 1 m sections and taken to
the GRAPE lab, where a geophysical log of the core is created. This device measures
geotechnical and geoacoustic properties of sediments in state, or while they are still
within the core casing. This immediate measure of such properties preserves several
sediment characteristics, as well as letting geologists view the inside of the core without
immediately splitting it open (Kayen et al., 1999). Three different measures are taken: 1)
soil seismic compression wave (p-wave) velocity; 2) soil wet bulk-density with gamma
rays and 3) sediment magnetic susceptibility. The first, p-wave velocity, can be used to
measure pore-water salinity, temperature and pressure within the core. The density
reading also helps characterize the core, as sand is denser than mud. These geophysical
logs (Fig. 7a, 7b & 7c) show distinct layering within core OO99-21, with the peaks
highlighting sandy layers.
$T
$T
$T
$T
$T
OO-99-21
OO-99-22
Figure 6: Gooseneck meander with core locations highlighted.
Core OO99-21 was chosen for sampling in this study because its position was
optimal for the study of layered sediment (Fig. 6) and the geophysical log displayed clear
layering. Once on land, the core was split vertically, photographed and described
(Appendix B). Subsamples were also taken for further analysis (Table 2). Grain size
analysis, XRD mineralogy, smear slides and Total Carbon Content analyses were
completed throughout the core. All sediment preparation methods are recorded in
Appendix A.
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Figure 7a: Velocity (Note: Peaks at the same depth are highlighted in the 3 geophysical logs.)
Figure 7b: Density
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Figure 7c: Magnetic Susceptibility
Results
Core OO99-21 was split vertically at MBARI in July 1999. Careful visual
documentation was kept of the core, including photographs and illustrations. These
images help to highlight specific features such as variations in grain size and layering,
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and are the map used to navigate through the core and determine where further analysis is
required. Sedimentary structures in core OO99-21 are an important indicator of the type
of sediment transport that is occurring and changes in energy and sedimentation rates at
the head of Monterey Canyon.
Sedimentary Structures: Energy is the Key
The tope of core OO99-21 begins with distinct variations in coloring, from light
medium gray to dark gray. Laminations occur throughout core OO99-21 and are also a
function of color variation, although the contrast is much finer. Laminations occur from
8-18 cm, 23 to 37 cm and 39 to 80 cm near the top of the core and then disappear from
the long clayey units until 450 to 476 cm. Laminations are approximately 5 per cm.
Several other color variation features are found in the core. The structureless clayey
layer from 227 to 301 cm has dark flecks with possible organic origin. Bioturbation may
be responsible for the dark patches. One other location within the core was a possible
candidate for bioturbated sediments at 430 cm.
Dark coloration of sediment has been associated with organic rich sediments in
marine settings. Monterey Bay is very fertile due to upwelling nutrients. This rich
environment has the potential to have very high rates of pelagic sedimentation.
Upwelling is dependent on several factors, some of which include seasonal winds,
currents and temperatures, causing variability in the primary productivity of Monterey
Bay. Dark and light banding resulting from organic content variability is possible in both

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Complete core diagrams are available in Appendix B.
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pelagic sedimentation and river flooding. Rivers transport high amounts of biologic
material during flood stage.
Depth in
Centimeters from
Core Top
Comments Depth in
Centimeters from
Core Top
Comments
0
♦•
247*
♦•
Silt and dark
material
3.5
♦•
292
Silt
19 303*
Sand
32 332*
Dark graded sand
38.5
338

Bioturbation in silt
50

357
♦•
Laminated silt
70
♦•
369
Silt with organics
86*
374*

Sand
126

397*
Sand
166*
♦•
410*
Silt
186*
428
♦•
Bioturbated silt
210
Silt
448*

Sand and silt
215

Dark lens
463
Silt
224*
Sand
487

Dark clay
227
♦•
Dark organic unit
*Samples used for grain size analysis.
♦Samples used for XRD mineralogy.
•Samples used for Total Carbon measurements.
Table 2: Samples from core OO99-21 used for analyses.
The faint laminations visible in core OO99-21 cm are very interesting features.
Laminations only form under very low energy conditions, especially when the
laminations are in clay or silt. The laminations are visible because of variations in the
coloring—alternating light and dark. This suggests an environment where there is some
variation in the sediment supply, causing the sediment to change color, yet has a fairly
low energy. This type of record could be left by the Salinas River, washing out
sediments in yearly high-water events, such as happens during the wet winters. Floods
carry lots of organic debris, causing the sediments deposited from flood events to be
darker. Fine sediment from the River could pass out into the ocean and then settle out
onto the sea floor below, which in this case was the flank of the Monterey Canyon.
Sand layers are found throughout the core and have a large variability in thickness
and character. The thick sand layers from 111 to 127 cm and 130 to 138 cm are very
well-sorted and show no grading. The contact between the sand and bottom and top clay
layers is sharp and clearly erosional. Sand layers and lenses such as at 23, 185, 227, 307,
336, 365, 380, 385, 395, 452, 460 and 477 cm exhibit graded contacts with layers above ,
while maintaining the sharp basal contact. The sharp contact is formed when the initial
energy erodes into the sediment surface before deposition begins. Grading is caused by a
gradual loss in transport energy, so that coarser grains fall out of suspension first,
followed by fines. These graded layers are much thinner, on the order of 3-8 cm, until
they are considered lenses with a thickness less than 1 cm.
Microscope Analysis
Defining the nature of the layers and laminations within the core is one of the
most important steps in determining where the sediment came from and what mode of
sediment transport deposited it. To do this several different analyses were completed.
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First, the core was subsampled at 29 different depths (Table 2). A smear slide was made
of each of these samples. Examining the sample under a petrographic microscope reveals
the minerals present as well as biogenic material. This may include diatoms and
foraminifera (Fig. 8a, 8b, 8c, 8d & 8e). Diatoms were the most common organism found
within core OO99-21. It is possible to use diatoms and foraminifera for dating sediment,
but it was not attempted in this study.
Figure 8a: Diatom at 50X, reference scale Figure 8b: Diatom at 50X
at top is ,34 mm.
Figure 8c
:
50X
Figure 8d: Polarized diatom 50X.Figure 8e: Diatom 50X
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Mineralogical analysis with the smear slides was only mildly successful as the
majority of the sediments were composed of various clays and silts, whose mineralogy
cannot be determined under a petrographic microscope. Quartz, biotite and chlorite
crystals were visible in many samples. Quartz crystals in the core were generally in the
very fine sand (63 µm) fraction or smaller, but were still angular. Comparing these
grains with the quartz from beach sand found right at the head of the canyon shows some
striking similarities (Fig. 9a, 9b & 9c). The beach sand is coarser but is just as angular.
Figure 9a: OO99-21-303 63 µm
Figure 9b: MBARI Beach 125 µm Figure 9c: OO99-21 125 µm
According to Clark and Osborne (1982), beaches in Monterey Bay are supplied through
long shore currents coming from the north. The sand from core OO99-21 has may have
the same source as the local beaches, and the finer grain size altered by increased
transportation.
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0%
20%
40%
60%
80%
100%
86 166 186 225 247 303 332 374 397 410 448 MBARI
Beach
Elkhorn
Kirby
Sample Depth/Location
Grain Size Analysis
(Corrected for floculation)
<38 µm
38 µm
68 µm
125 µm
250 µm
500 µm
1000 µm
>2000 µm
Figure 10: Grain size distribution for core OO99-21 and local sands.
Grain Size Analysis
Grain size analysis was done on 11 samples, some predominantly clay and silt and
others sandy layers (Table 2, Fig. 10). Apparent first is the overall fine grain-size within
the core. The majority of the sediment is clay and silt fractions (38 µm and <38 µm),
with the occasional very fine to fine sand layer (63 µm and 125 µm.) Of the sampled
layers, ones at 86, 166, 303 and 397 were >75% fraction of very fine sand (63 µm) and
fine sand (125
NAME Millimeters Micrometers
Boulder 4096
Cobble 256
Pebble 64
Gravel
Granule 4
Very coarse sand 2
Coarse sand 1
Medium sand 0.5 500
Sand
Fine sand 0.25 250
Very fine sand 0.125 125
Coarse silt 0.062 31
Medium silt 0.031 16
Fine silt 0.016 8
Silt
Very fine silt 0.008
Clay
↓ ↓
Table 2: Standard Grain Size Chart
µm.) The grain size percentages were similar between all four of the medium to fine sand
layers. The sandy layers are highly micaceous (Fig. 8c). Monterey mass granite and Ben
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Lomond granite contain biotite, and are in the source areas for both the Salinas and
Pajaro Rivers.

Mixed very fine sand and clay layers at 225, 332, 374 and 448 cm
contained greater variation in the 63 µm, 38 µm and <38 µm fractions than did the
coarser layers between larger grain size fractions. Coarse silt and finer dominated in
samples from 186, 247 and 410 cm.
XRD Analysis
The sedimentary features in the very fine-grained portions of Core OO99-21 are
important to decipher as well. With silt or clay fractions, it is not possible to analyze
mineralogy with a petrographic microscope. Instead, an X-ray Diffraction (XRD)
Microscope,
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was used for analysis of 8 samples from the core (Table 2). This type of
analysis works by sending a x-ray across the surface of a fine power at many different
angles. A relative value of the x-ray return is measured at each angle. Thousands of
minerals, natural and synthetic, have been measured this way and the results are
published in (Mineral Powder Diffraction file, JCPDS). A computer program compares
the measured peak values at different angles with known peak values and matches the
sample with a mineral. For this to work, the sample should consist of only one mineral.
Separating clay minerals is a very involved process, so was not attempted. The results
for the XRD scans (Table 3) show that most of the samples were Quartz, SiO
2
. The
lower the Assurance Values from Table 3, the higher the correlation between the sample
peaks and the known signature values for Quartz peaks (Fig. 11a & 11b). A granitic
source for these clays can be found in the Monterey mass granite or the Ben Lomond
granite, found in the drainage systems of the Salinas and Pajaro Rivers.
Sample
Number
26.650
Ø
Peak
Value
Highest
Peak
Second
Highest
Peak
Second
Highest
Peak Value
Peak
Finding
Program
Mineral 1
Assurance
Values
(OP)
Peak Finding
Program
Mineral 1
Assurance
Values
(OP)
Quartz 100
25.650
Ø
50.150
Ø
14.35 Quartz, Syn.59 Quartz, Low.85
OO99-21-0 100
25.650
Ø
27.700
Ø
25.66 Quartz, Low 143.23 Quartz, Syn 167.65
OO99-21-3.5 100
25.650
Ø
20.950
Ø
27.59 Quartz, Low 5.58 Quartz, Syn 7.67
OO00-21-70 100
25.650
Ø
28.150
Ø
27.03 No Match No Match
OO99-21-166 100
25.650
Ø
25.650
Ø
33.76 Quartz, Low 104.80 Quartz, Syn 108.22
OO99-21-227 100
25.650
Ø
21.000
Ø
33.63 Quartz, Syn 6.79 Quartz, Low 8.77
OO99-21-247 100
25.650
Ø
21.100
Ø
18.79 Quartz, Syn 6.72 Quartz, Low 6.87
OO99-21-357 100
25.650
Ø
28.000
Ø
24.78 Quartz, Syn 4.41 Quartz, Low 6.30
OO99-21-428 100
25.650
Ø
21.000
Ø
26.19 Quartz, Low 4.94 Quartz, Syn 5.07
Table 3: XRD Microscope results
Several of the secondary peaks were not Quartz peaks, suggesting the possibility
of other clay minerals mixing with the Quartz. It is possible that Kaolinte or Illite (Fig
11c) also make up part of the clay fraction. For this analysis the Quartz peaks were
removed and all secondary peaks highlighted and then the sample reanalyzed with the
Peak Finding/Matching program. This approach was non-conclusive, as no matches were
found, probably indicating that more than one mineral was responsible for the peaks.

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Stanford University Mineralogy Lab, Stanford University, CA.
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Figure 11a: XRD Microscope data graph for sample depth 3.5 cm. The blue line represents the
sample from 247 cm, the red peaks are Quartz.
Figure 11b: XRD Microscope data graph for sample depth 357 cm. Note the different scale,
but the Quartz peaks are responsible for many of the sample peaks, except the one at 28.
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Figure 11c: XRD Microscope data graph. The blue line represents the sample from 247 cm, the red
peaks are Quartz and yellow are Illite. Note the peak close match between Quartz peaks and sample
peaks, except at 28, which could be a clay mineral peak.
Total Carbon Content
Sample
Depth
WT%N WT%C
δ
15
N
AIR
δ
13
C
PDB
0
0.09 0.96 4.59 -21.78
36
0.13 1.62 4.51 -21.88
50
0.10 1.12 4.82 -21.44
70
0.11 1.08 4.76 -22.14
126
0.03 0.31 3.73 -21.30
166
0.12 1.41 4.67 -21.80
215
0.09 1.10 4.81 -22.09
227
0.08 0.79 5.42 -21.08
247
0.10 1.30 4.47 -21.53
338
0.12 1.19 5.25 -22.01
357
0.11 1.11 5.18 -22.30
374
0.08 0.77 4.64 -21.62
428
0.12 1.18 4.71 -22.31
448
0.10 0.98 5.30 -21.46
487
0.10 1.12 5.22 -21.64
Table : Total carbon content results*.
*
Further analysis of the total carbon content results is required and will be completed with the collaboration
of Charlie Paull at a future date.
Discussion
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Sediment Supply to the Monterey Canyon Head
The primary sediment supply to the Monterey Canyon is from the local Salinas
and Pajaro Rivers (Griggs and Hein, 1980). Long-shore drift supplies some sediment to
Monterey Bay, but compared to sediment input from riverine sources, this source is
insignificant (Clark and Osborne, 1982). The Pajaro River is north of the Monterey
Canyon head, and the Salinas River lies just south. Both of these rivers supply a
significant amount of sediment to the bay annually (Table 4). Winter flooding caused by
heavy rains causes the vast majority of the sediment discharge and events which can
leave plumes of sediment on the surface of the bay 60 km seaward (Griggs and Hein,
1980).
Canyon Sedimentation
Core OO99-21 tells a fascinating tale about the sedimentation in Monterey
Canyon. The Monterey Canyon appears to be actively accumulating sediment in its
upper reaches. The small grain size of the sediment suggests that fine sediment
River Drainage Area Period of Record Average Sediment
Yield (Metric
Tons/sq. km./Year)
Average
Sediment Load
(Metric
Tons/Year)
Salinas River 10764 1967-1977 130 1400000
Pajaro River 3144 70 220000
*U.S. Geological Survey—Water Data Reports, 1970-75.
Table 4: Sediment Discharge Data for Salinas and Pajaro Rives (from Griggs and Hein, 1980).*
settles out of suspension in a relatively quiet environment, one in which canyon infilling
and sediment accumulation could occur. The classical picture of submarine canyon
sediment transport is concerned with turbidity currents, high energy events moving from
the head of the canyon to the far reaches of the fan. The sediment record presented in
core OO99-21 demonstrates that there is a quiet water environment in the upper reaches
of Monterey Canyon. In a similar study of Hudson Canyon, Drake et al. (1978)
concluded that at the head of the canyon in 430 meters of water, clayey sediment was
being deposited and transport by high-velocity turbidity currents was negligible. The
circumstances and conclusions of this report were incredibly similar to the current study.
The creation of such a low energy environment in the Monterey Canyon presents
many fascinating questions. The circulation of Monterey Bay is well studied and the
nature of currents and upwelling occurring in the Bay throughout the year has been
explored by Breaker & Broenkow (1994), Shepard et al. (1979), Bigelow and Leslie
(1930), and Skogsberg (1936.) Surface currents off the mouth of the Salinas River
flowed northward 65% and southward 35%, as shown by current meter data from January
1976 to January 1977 (Breaker & Broenkow, 1994.) Currents from the Pajaro River were
also measured from December 1979 to December 1980 and found to flow to northwest at
speeds of 6 to 13 cm/sec, with a reversal occurring in the spring (Breaker & Broenkow,
1994.) The Salinas and Pajaro River are both possible sediment supplies for the
Monterey Canyon. Sediment from the Salinas is dumped directly into the canyon head at
the jetties at the mouth of Elkhorn Slough. The Pajaro would supply sediment during the
spring reversal.
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Figure 12: Schematic Diagram of Monterey Canyon Currents
Breaker & Broenkow (1994) also studied currents within the Monterey Canyon
and found that near the head of the canyon, specifically between the depths of 384 and
155 m, up-canyon currents dominated. These bottom currents have been known to
achieve speeds of 25 cm/sec (Breaker & Broenkow, 1994), fast enough to suspend clay
sediments, unless cohesion is very strong. Turbid layers from the northward flowing
Salinas River discharge and the up-canyon currents could both be contributing to the
sedimentation near Gooseneck meander. Possibly a null between these two currents is
creating a quiet water environment for deposition.
The sand has several possible sources. Sands from Elkhorn Slough and the
MBARI beach are significantly coarser than the sand layers from core OO99-21.
(Fig.) A possible source for the sandy layers is the shelf itself. Hunter et al. (1987)
studied the near shore shelf sands and found that of 37 samples collected, 21 were fine
sand (.250 -.125 mm or 250-125 µm) and 10 more were fine to medium sand (.354-.250
mm or 400-250 µm.) Approximately 85% of the shelf samples collected were fine-
grained sands, much closer to the grain size in the sand layers of the core than the beach
sands. One possible explanation for the sand units could be that storms, such as hit the
coast every 50, 100 or even 200 years, could disturb the shelf sediments. Griggs & Hein
(1980) state that the dominant process for suspending fine-grained sediments was
nearshore wave turbulence, as would occur during a storm. Drake et al. (1978) also
concluded that storm induced wave activity on the shelf suspended sediment that resulted
in significant sedimentation near the head of the Hudson Canyon. Okey (1993) studied
sediment turbidity as related to benthic communities in the near shore and Monterey
Canyon head areas, and observed that storm events were the most disruptive events to the
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sediment. Sediments disturbed by wave action would settle out of the water column once
the storm energy had dissipated. Depending on the nature of the shelf sand, the sand
layer would be graded if a great variation in grain size existed on the shelf; or would have
no grading if all the shelf sand was the same size.
Figure 13: Captured image from interactive 3D perspective of
Gooseneck Meander showing channel and flank.
One further problem with using the classical turbidity current theories of
submarine canyon transport in this study is the core’s location at the head of the
Monterey Canyon. The core was drilled at a depth of 380 meters of water. Turbidity
currents usually begin with a slope failure or mass wasting at this depth, so one would not
expect to find deposits from turbidity currents that would start to form at this depth.
Also, this core was taken on top of a raised canyon flank, about 100 meters off of the
floor of the canyon (Fig. 13). Turbidity currents would not be depositing material this
high up on the canyon wall this close to their source. We finally come to the obvious
problem presented by quiet water deposition along an exposed wall of layered sediments.
Several possible explanations exist for this occurrence. With quiet water deposition,
sediment draping occurs and layers of sediment evenly coat the entire exposed surface.
Along the edge of the canyon wall the sediment layers would be unstable because of
steep slopes and slumping down the canyon wall would occur, exposing a fresh surface
periodically. Occasional high energy events in the thalwag of the canyon would erode
into the main channel and, depending on the size of the event, possibly the canyon wall,
without disturbing sediment accumulation on the flank. Down-stream high energy
currents and the more common up-stream currents would both disrupt sediment in the
canyon channel, possibly causing sedimentation up on the canyon flank.
Recommendations
Further exploration into the specifics of submarine sedimentation in the Monterey
Canyon would be valuable information. Monterey Canyon lies in the heart of an
intensely studied marine environment. Biologists, geologists, and chemists are building
an impressive body of knowledge about this system. The geologic processes at work in
the canyon are important pieces of this knowledge.
19
Further analysis of core OO99-21 should include carbon dating, especially as the
preliminary information about carbon content is already available. Dating the sediment
will place some chronological boundaries on the core. It would be possible to determine
sedimentation rates for the duration of the core. Multiple cores along the axis of the
canyon, near the head and further down canyon, would establish a more complete view of
total canyon activity.
Sedimentation in Monterey Canyon may be approached by several other methods.
Multiple studies of submarine canyon currents have been done (Keller & Shepard, 1978 ,
Kinoshita and Noble, 1995 and Drake et al., 1978) using current meters placed along the
canyon axis and flank. Nephlometers are used to measure turbidity, and could be used
stationed along the upper reaches of Monterey Canyon to define the variations in
turbidity as a function of seasonal and storm events. The U.S. Geological Survey has
attempted a similar study for sedimentation deep (>1400 m) in the canyon. A
comparison of this study (Noble et al., 1994) with data on sedimentation at the head
would be a very interesting project.
Acknowledgments
This project was completed with the help of many people. My intern advisors at
MBARI, Gary Greene and Norman Maher, provided excellent assistance and advice with
all aspects of the project. I adopted Charlie Paull as another advisor, and he provided
frequent bursts of new ideas and energy during the summer. Much of the analysis
included in this paper was done through the assistance of others. I would like to thank
Homa Lee and Brian Edwards from the U.S. Geological Survey for their work on the
Ocean Olympus cruise with the geophysical logs. Bob Jones certified me to work on the
XRD Microscope at Stanford University. Rob Dunbar made it possible for the Total
Carbon content analysis to be run on the samples and David Mucciarone ran analyses for
me at the Stable Isotope Lab at Stanford. Many heart-felt thanks are extended to George
Matsumoto for making this internship not only possible but one of the greatest research
experiences of my academic career.
20
References
Bigelow, H.B. and Leslie, M. 1930. Reconnaissance of the Waters and Plankton of Monterey Bay, July,
1928. Bulletin of the Museum of Cooperative Zoology, Harvard College, vol. 70, p. 427-581.
Bouma, A.H., 1965. Sedimentary Characteristics of some Samples Collected from Submarine Canyons.
Marine Geology, vol. 3, p. 291-320.
Breaker, L.C. and Broenkow, W.W., 1994. The Circulation of Monterey Bay and Related Processes.
Oceanography and Marine Biology: an Annual Review, vol. 32, p. 1-64.
Clark, R.A. and Osborne, R. H., 1982. Contribution of Salinas River Sand to the Beaches of Monterey
Bay, California, During the 1978 Flood Period: Fourier Grain-Shape Analysis. Journal of Sedimentary
Petrology, vol. 52, no. 3, p. 807-822.
Drake, D.E., Hatcher, P.G. and Keller, G.H., 1978. Suspended Particulate Matter and Mud Deposition in
Upper Hudson Submarine Canyon. In
Sedimentation in Submarine Canyons, Fans, and Trenches
, eds.
Stanley, D.J. and Kelling, G., Dowden, Hutchinson & Ross, Inc., 1978, p. 33-41.
Greene, H.G., Clarke, S.H. and Kennedy, M.P., 1991. Tectonic Evolution of Submarine Canyons Along
the California Continental Margin. From Shoreline to Abyss, Special Publication No. 46, Society for
Sedimentary Geology, p. 231-248.
Greene, H.G., Stubblefield, W.L. and Thebridge, JR., A.E., 1989. Geology of the Monterey Submarine
Canyon System and Adjacent Areas, Offshore Central California. U.S. Geological Survey, Open-File
Report 89-221.
Griggs, G.B. and Hein, J.R., 1980. Journal of Geology, vol. 88, p. 541-566.
Hampton, L.A. and Garrett, P., 1977. The Role of Subaqueous Debris Flow in Generating Turbidity
Currents. Journal of Sedimentary Petrology, vol. 42, p. 775-793.
Hicks, D. M., and Inman, D.L., 1987. Sand Dispersion from an Ephemeral River Delta on the Central
California Coast. Marine Geology, vol. 77, p. 305-318.
Hunter, R.E., Dingler, J.R., Anima, R.J. and Richmond, B. M., 1988. Coarse Sediment Bands on the Inner
Shelf of Southern Monterey Bay, California. Marine Geology, vol. 80, p. 81-98.
Keller, G.H. and Shepard, F.P., 1978. Currents and Sedimentary Processes in Submarine Canyons off the
Northeast United States. in
Sedimentation in Submarine Canyons, Fans, and Trenches
, eds. Stanley, D.J.
and Kelling, G., Dowden, Hutchinson & Ross, Inc., 1978, p. 15-32.
Kinoshita, K. and Noble, M., 1995. The Monterey Submarine Canyon, California Moored Array Data
Report. U. S. Geological Survey Open-File Report 95-838.
Klein, G. DeV., 1980. Sandstone Depositional Models for Exploration of Fossil Fuels, 2
nd
ed. CEPCO
Division, Burgess, Minneapolis, Minn., p. 149.
Mineral Powder Diffraction File, JCPDS, International Center for Diffraction Data. Swathmore, 1993.
Noble, M.A., Pilskaln, C.H., Rosenfield, L.K. and Schwing, F., 1994. Currents and Sediment Movement in
Monterey Submarine Canyon. 1994 Eastern Pacific Ocean Conference, Mt. Hood, Oregon. Abstract.
Okey, T.A., 1993. Natural Disturbances and Benthic Communities in Monterey Canyon Head. Masters
thesis at Moss Landing Marine Laboratories.
Shepard, F.P. and Marshall, N.F., 1978. Currents in Submarine Canyons and Other Sea Valleys. In
Sedimentation in Submarine Canyons, Fans, and Trenches
, eds. Stanley, D.J. and Kelling, G., Dowden,
Hutchinson & Ross, Inc., 1978, p. 3-14.
21
Skosber, T. 1936. Hydrography of Monterey Bay, California. Thermal conditions, 1929-1933.
Transactions of the American Philosophical Society vol. 29, p. 152.
Stow, D.A.V., 1985. Deep-sea Clastic Sediments, in Brenchley, P.J. and Williams, B.P. J. (eds.),
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Blackwell, London, p. 67-93.
22
Appendix A
Smear Slide Preparation
Smear slides were created using a very small amount of sediment from each of the
29 samples collected from core OO99-21. This sediment was mixed with de-ionized
water, spread onto a glass slide and dried on a hot plate. The slide surface was then
saturated with Nortion Optical Adhesive 41 and a cover plate was placed on top. The
slide cured for 30 minutes under a UV-light source. All slides were made from samples
untreated for organic content or grain size.
XRD Sample Preparation
Clay samples were taken from the above 29 core OO99-21 sub-samples. XRD
analysis requires very fine powdered fractions, preferrably mono-mineralic, so only the
clay fraction was used for this analysis. (Also, the variations in color causing the faint
laminations in the clay units was of specific interest, and as the cause of these laminations
was not visible in the smear slides, it was hoped that XRD analysis might find some
difference.)
Total Carbon Content Sample Preparation
Total carbon and nitrogen samples were run by David Mucciarone in Rob
Dunbar’s Stable Isotope lab at Stanford University. The only preparation done prior to
sending them to the lab was drying and powdering.
Gamma Ray (GRAPE) Analysis of Sediment Core
This analysis was performed immediately after the core was collected at sea by
trained technicians and scientists with the U.S. Geological Survey.
Grain Size Analysis Sample Preparation
The following method was adapted from Folk (1974) and used in the sediment lab
of Greene, at Moss Landing Marine Laboratories. The procedure works well for the sand
fraction, but with the very fine sand and clays of core OO99-21, there were several
problems. The multiple steps involving watering and drying the sample guaranteed that
some of the fine fraction would be lost to containers and spillage throughout the process.
In fact if the percentages of sample lost are examined compared to the grain size of the
sample, it can be seen that the finer majority of the grains, the larger the losses.
Floculation was a significant factor in samples that were primarily clay and silt, skewing
some of the weight percentages for the coarser fraction. This was accounted for after the
initial analysis by adding the floculated clay weight fractions to the clay fraction.
1.

Sediment sample was mixed with 10 mL of hydrogen peroxide and boiled for
4 hours to remove organic material. Then samples were dried overnight at
105 C.
2.

Sample was re-hydrated and mixed with 10 mL of 10 molar Calgon© solution
to prevent floculation of clay fraction.
3.

Sample was wet sieved into <63 µm (clay and silt) and >63 µm (sand)
fractions on a (Brand) Ro-Tap for 15 minutes. These fractions were placed in
pre-weighed containers and dried overnight in a 105 C oven.
4.

Samples were weighed after all water was removed and then prepared for dry
seiving by breaking apart all floculation with a mortar and pestle. Dry sieving
23
was done on the Ro-Tap for 15 minutes and broke the sediment down to 8
different fractions (Table 1) and placed on pre-weighed containers.
Pan #
µm
Pan #
µm
10 120 125
18 1000 230 63
35 500 400 38
60 250 Pan <38
Table 1: Dry sieving grain size fractions.